TANGO · It takes three to tango: semiconductors with three bands for a new generation of optoelectronic devices

Semiconductors, halfway between metals and insulators, are at the heart of the optoelectronic applications that have revolutionized our world for the last 70 years. Their hallmark characteristic is the existence of two electronic bands—the conduction and valence bands (CB and VB)—separated by a forbidden energy gap, which enables electrical and optical access to two distinct electronic populations. 3-band materials (3BM) represent a new class of semiconductor featuring three electronic bands and three energy gaps (one between each pair of bands). The driving force behind two decades of 3BM research has been the development of high-efficiency solar cells. Yet, all the attempts to fabricate an operational 3-band solar cell either failed or proved to be unpractical mainly due to a fundamental reason: the impossibility of the 3BM candidates to maintain three distinct electronic populations at room temperature (RT), due to strong interband thermal coupling. TANGO aims to overcome this hurdle by implementing an innovative band-engineering strategy. A family of functional 3BMs will be achieved via precise tailoring of the band structure of hybrid heterocrystals comprising quantum dots (QD) in a perovskite matrix (QDIP). In this approach, the QD electronic states give rise to a third band thermally decoupled from the VB and CB even at RT. The successful implementation of QDIP-based 3BMs (Objective 1) will permit the fabrication of the first operational 3-band solar cell (Objective 2), setting a new landmark that could lead to cheap high-efficiency photovoltaics.Beyond solar cells, the unique properties of 3BMs, which allow for external access to three electronic populations, will be leveraged to develop a groundbreaking device: the 3-band transistor (Objective 3), able to switch among three logic states. The demonstration of such a 3-state element would imply a breakthrough in the field of multivalued digital logic, which promises enhanced computation and storage capacity.